1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, which varies the cam phase of at least one of an intake cam and an exhaust cam, relative to a crankshaft of the engine, to thereby control valve timing, and more particularly to a valve timing control system that executes cleaning to prevent undesired locking of a cam phase-varying device for varying the cam phase.
2. Description of the Prior Art
A valve timing control system of this kind controls valve timing for opening and closing an intake valve and/or an exhaust valve and a valve overlap between the intake and the exhaust valve, by varying the cam phase, thereby controlling the charging efficiency and internal EGR of the engine, with a view to improving the power output thereof and reducing exhaust emissions therefrom. The valve timing control system is disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2000-104571. In this control system, a cam phase-varying mechanism supplied with oil pressure which is controlled by an oil pressure control valve varies the cam phase of the intake cam by changing the angle of the intake cam relative to the crankshaft. Further, in this control system, to prevent undesired locking of the oil pressure control valve and the like due to biting of a foreign matter, cleaning is carried out in which the cam phase-varying mechanism is forcibly reciprocated between the most advanced position and the most retarded position. This cleaning is performed over a limited time period for deceleration fuel cut-off operation during which the throttle valve is fully closed, more specifically, immediately after transition of engine operation to the deceleration fuel cut-off operation.
In this control system, however, since the cleaning is started simultaneously with the start of the deceleration fuel cut-off operation, the intake valve is actuated to the most advanced position for execution of the cleaning immediately after the intake pipe pressure has been changed in a negative pressure-increasing direction due to the fully-closed state of the throttle valve. This produces a large valve overlap between the intake valve and the exhaust valve, so that positive pressure is introduced into the intake pipe via the exhaust and intake valves made open in an overlapping fashion, which changes the intake pipe pressure toward the positive pressure side. Thus, at the start of the cleaning, the intake pipe pressure is steeply changed toward the positive pressure side immediately after the preceding increase in negative pressure. This pressure reaction increases the magnitude of an aftershock subsequent to a deceleration shock, causing the driver to feel a large shock, which impairs drivability.
It is an object of the invention to provide a valve timing control system for an internal combustion engine which is capable of reducing an aftershock subsequent to a deceleration shock, for which the execution of the cleaning is responsible, and increasing the frequency of execution of the cleaning.
To attain the above object, the present invention provides a valve timing control system for an internal combustion engine, for controlling valve timing for opening and closing at least one of an intake valve and an exhaust valve by changing a cam phase which is a phase of at least one of an intake cam and an exhaust cam, relative to the crankshaft, the engine having a camshaft on which at least one of the intake cam and the exhaust cam is arranged.
The valve timing control system is characterized by comprising:
a cam phase-varying device for varying the cam phase by rotating the camshaft relative to the crankshaft;
deceleration fuel cut-off operation-determining means for determining whether or not the engine is performing a fuel cut-off operation in which supply of fuel to the engine is stopped during deceleration; and
cleaning control means for forcibly driving the cam phase-varying device within a predetermined cam phase range when a predetermined delay time has elapsed after the deceleration fuel cut-off operation is started, to thereby prevent undesired locking of the cam phase-varying device.
According to the valve timing control system for an internal combustion engine, the cam phase-varying device causes the camshaft to rotate relative to the crankshaft to change the cam phase of an intake cam and/or an exhaust cam, whereby the valve timing for opening and closing an intake valve and/or an exhaust valve is controlled. Further, cleaning in which the cam phase-varying device is forcibly driven within a predetermined cam phase range is carried out when a predetermined time period has elapsed after the deceleration fuel cut-off operation was started. Thus, the cleaning is not carried out before the predetermined time period has elapsed after the start of the deceleration fuel cut-off operation. This causes the intake pipe pressure, which has been changed in a negative pressure-increasing direction due to full closing of the throttle valve immediately before the start of the deceleration fuel cut-off operation, to be increased toward the positive pressure side to some extent and made stable by the time the cleaning is started. Therefore, even if there occurs a large valve overlap between the intake valve and the exhaust valve due to subsequent execution of the cleaning, causing the positive pressure to be introduced into the intake pipe, the amount of change in the intake pipe pressure is small. Thus, a change of the intake pipe pressure in the negative pressure-increasing direction caused by the full closing of the throttle valve and a change of the same toward the positive pressure side caused by execution of the cleaning are produced in a distributed fashion with an appropriate shift in timing. This makes it possible to reduce the magnitude of an aftershock subsequent to the deceleration shock and makes it difficult to be felt by the driver as a shock, thereby improving drivability.
The engine has a transmission connected thereto, and preferably, the valve timing control system further comprises gear ratio-detecting means for detecting a gear ratio of the transmission, and delay time-setting means for setting the delay time to a smaller value as the detected gear ratio is smaller.
In general, the deceleration fuel cut-off operation terminates in a shorter time period as the gear ratio of the transmission is smaller since the engine brake is more effectively applied with a smaller gear ratio. Therefore, according to this preferred embodiment, since the delay time as a waiting time period before execution of the cleaning is set as described above, it is possible to appropriately secure the chance of execution of the cleaning, and increase the frequency of the same.
More preferably, the delay time-setting means sets the delay time to a smaller value when the detected gear ratio has a smallest value than when the detected gear ratio has a value other than the smallest value.
According to this preferred embodiment, even when the transmission is set to the smallest gear ratio in which the deceleration fuel cut-off operation terminates in a shortest time period, it is possible to appropriately secure the chance of execution of the cleaning.